Coated Steel Sheet or Strip

ABSTRACT

The invention relates to a coated steel sheet or strip with a ground coating made of steel, onto at least one upper side of which a coating is applied by hot-dip galvanizing, the coating being formed from a melt consisting of 0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, the remainder being zinc and unavoidable impurities, and, with a coating thickness of a maximum of 3.5 μm on each side and a coating weight of a maximum 25 g/m 2  on each side, guarantees that the steel sheet, in the salt spray mist test carried out in accordance with DIN 50021-SS, shows the first formation of red rust at the earliest after 250 hours. With such a sheet or strip, a flat steel product is provided which possesses an optimum combination of high corrosion resistance and optimum weldability and which is particularly well-suited for use as a material for motor vehicle chassis construction or for the construction of domestic appliances.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of International Application No. PCT/EP2006/050955, filed Feb. 15, 2006, which claims the benefit of and priority to European Application No. 05003762.1, filed Feb. 22, 2005, which is owned by the assignee of the instant application. The disclosure of each of the above applications is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a coated steel sheet or strip, with a ground coating made of steel, onto at least one upper side of which a zinc coating is applied by hot-dip galvanizing.

BACKGROUND

On steel sheets or steel strips of this type the zinc coating ensures a high degree of corrosion resistance. The thicker the coating is, the greater the resistance. Accordingly, with a conventionally alloyed zinc coating, in a spray test according to DIN 50021 carried out on a bright, unpainted specimen, with a coating of 25 g/m², red rust already occurs after 24 hours, while with a coating of 70 g/m red rust does not occur until after 120 hours.

The thickness of the coating required with the prior art for adequate corrosion resistance does, however, incur problems with regard to weldability. This applies in particular if the intention is that through-welding is to be produced by means of laser welding in the overlap joint without any joint gaps at high welding speeds, such as is required in the motor vehicle chassis construction sector or the domestic engineering sector. The seam produced by such welding should be free of passage holes, should be largely free of cratering, and should not have any open pores.

One possibility for manufacturing hot-dip galvanized steel sheets with increased corrosion resistance with simultaneously reduced coating weight is described in EP 0 038 904 B1. According to this prior art, a zinc coating containing 0.2% by weight Al and 0.5% by weight Mg is applied on a steel substrate by hot-dip galvanizing. As a result of the magnesium content, the hot-dip galvanized steel strip obtained in this way, with a coating weight of 44 g/m² per side, in the painted state, had first rust formation only after a spray duration of more than 2,000 hours under a salt spray test in which the individual specimen was sprayed with an NaCl solution under the conditions specified in Japanese Industrial Standard JIS Z 2371. This long period before the occurrence of rust was achieved thanks to the combined corrosion protection of the zinc coating and painting.

Despite the reduction in the coating weight achieved in accordance with EP 0 038 904 B1 and the simultaneous good corrosion resistance, the hot-dip galvanized steel sheets produced in this way still do not fulfil the requirements imposed in the motor vehicle construction sector with regard to weldability.

SUMMARY OF THE INVENTION

The invention, in one embodiment, features a flat steel product which possesses an optimum combination of high corrosion resistance and optimum weldability and which is particularly well-suited for use as a material for motor vehicle chassis construction or for the production of domestic appliances.

In one aspect, the invention features a coated steel sheet or strip, which has a ground coating made of steel, onto at least one upper side of which a coating is applied by hot-dip galvanizing, the coating being formed from a melt consisting of 0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, the remainder being zinc and unavoidable impurities, and on each side, with a coating thickness of a maximum of 3.5 μm and a coating weight of a maximum of 25 g/m², guarantees that the steel sheet, in the salt spray mist test carried out in accordance with DIN 50021-SS, shows the first formation of red rust at the earliest after 250 hours.

DESCRIPTION OF THE INVENTION

A hot-dip galvanized flat steel product according to the invention possesses surprisingly good corrosion resistance with a coating weight minimised in relation to the prior art coating weight of a maximum of 25 g/m² on each side. The low coating weight and the low thickness of the coating associated with this, of a maximum of 3.5 μm on each side, in combination with the high corrosion resistance, makes sheet or strip according to the invention particularly well-suited for the production of components which are manufactured by the welding of individual sheet elements. Accordingly, with steel sheets produced in accordance with the invention, elements for motor vehicle chassis or domestic appliance technology can be manufactured in particular, in that the individual sheet components formed from sheet or strip according to the invention can be welded to one another by laser beam welding at high welding speeds economically and with optimum results.

The corrosion resistance according to the invention is determined on the basis of a salt spray mist test in accordance with DIN 50021-SS in a corrosion short-term test process on bright unpainted steel sheet, in which a neutral 5% NaCl solution, as the corrosive agent, is sprayed continuously at a temperature of 35±2° C. in a chamber. The steel sheet samples are in this situation placed in the chamber at an angle of inclination to the horizontal of 65 to 75°. In the practical test carried out in this manner, it has been demonstrated that sheets and strips coated in accordance with the invention regularly do not show any red rust formation until after a test duration of 300 hours.

The magnesium content in the melt intended for the coating remains essentially unchanged in the coating. The Al content of the coating, in the finished steel strip according to the invention, by contrast, is as a rule 1.8 to 3.2 and in particular 2 to 3 times higher than in the melt. An optimum corrosion protection is attained when the coating has an Mg content of 0.4-1.0% by weight, in particular at least 0.5% by weight.

If it is intended that the coating of the steel base material is to be carried out in the galvanealed process, the melt contains preferably less than 0.15% by weight of aluminium. Al contents of the melt which are suitable for standard practice are in this case in the range of 0.12-0.14% by weight.

If, by contrast, a conventionally hot-dip galvanized steel sheet according to the invention is provided, the Al content of the melt is preferably at least 0.15% by weight.

A further surprising property which makes a flat product according to the invention particularly suitable for use in chassis construction becomes apparent when such a sheet or strip is painted. Accordingly, a mandrel bend test carried out on the basis of DIN EN ISO 6860 for sheets or strips according to the invention at room temperature and at −20° C. produces good paint adherence capacity. In particular, at a temperature of −20° C. there is no indication of paint flaking or of flaking of the coating from the base material.

For the test carried out to determine the paint adherence capacity, a full paint structure was applied onto a steel sheet specimen after alkaline cleaning and phosphating, this structure comprising a 20 μm thick cathodic dip paint coating, a 32 μm thick filler paint coating applied onto this and a 40 μm thick base coating. The bending carried out over the conical mandrel did not lead to any detachment of the paint coating at room temperature or at −20° C.

In addition to a high corrosion resistance and a good paint adherence capacity, sheets or strips according to the invention have outstandingly good resistance to stone impact. Thus, for example, in the stone impact test carried out in accordance with DIN 65996-1B, it was possible to demonstrate that, with steel sheets according to the invention, stone impact did not cause any flaking of the coating from the base coat.

To manufacture sheets according to the invention, a fine steel strip is subjected to a continuous hot-dip galvanizing process in a galvanizing plant operating at a strip speed of a typical 60 to 150 m/min. To do this, the sheet or strip to be galvanized is firstly annealed in a furnace, such as a DFF furnace (Direct Fired Furnace) or, preferably, an RTF furnace (Radiant Type Furnace). Following on from the furnace, the sheet or strip runs through the reduction furnace section, in which it is held under a protective gas atmosphere with 3.5-75% hydrogen. The temperatures attained in the course of the annealing lie in the range from 720-850° C.

The sheet or strip annealed in this way is then conducted via what is referred to as a nozzle, with the exclusion of air, into the zinc bath, which is formed from a melt containing 0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, in particular 0.4-1.0% by weight and 0.5-1.0% by weight respectively, the remainder being zinc and unavoidable impurities.

After the sheet or strip emerges from the melt bath, the thickness of the coating is restricted in an inherently known manner by means of stripper nozzles to a value of a maximum of 3.5 μm on each side, with the result that, with the flat product obtained according to the invention, the coating weight is restricted to a maximum of 25 g/m² per side.

In order to prevent an over-proportionate formation of slags and inter-metallic phases on the melt bath, it may be expedient to conduct an inert gas flow over the surface of the bath. This inert gas flow can be derived from the stripper nozzles which are used to adjust the thickness of the coating or can be supplied from separate nozzles, which distribute the inert gas in mist fashion over the surface of the bath. As an alternative, the entire melt bath can be surrounded by a housing enclosure, in which an inert atmosphere is maintained. Especially suitable as the inert gas for this purpose is nitrogen.

The slag formation can also be reduced by adjusting the bath temperature to a range of 380-450° C. For the same purpose, the temperature of the strip can be restricted on immersion to 360-500° C. in order in particular to minimise the inclination to oxidise in the immersion area.

After emerging from the melt bath the coated strip is cooled at a cooling speed of at least 10 K/s.

By means of subsequent in-line rolling at rolling degrees of 0.3-1.5%, if required, the desired texturing of the surface can then be carried out.

Inasmuch as the coated is likewise subjected in-line to subsequent heating in the temperature range of 300-600° C., either a redistribution takes place inside the ZnMg coating or through-alloying is achieved into a ZnFeMg coating. The melts used to produce such a coating preferably have an Al content of less than 0.15% by weight, and in particular 0.12-0.14% by weight.

To extend the range of application, it is also possible then to apply in an inherently known manner a thin-film layer on the coating.

The effects achieved by the invention have been confirmed on the basis of an experiment, in which a steel strip, 0.82 mm thick, hard-rolled from conventional IF steel, was initially subjected to an alkaline spray cleaning, a brush cleaning, and an electrolytic cleaning.

This was then followed by annealing, during which the cleaned steel strip was annealed under protective gas (5% H₂, remainder N₂) to a temperature of 800° C. The annealing time was 60 s.

The steel strip annealed in this way is then cooled, such that it is immersed at a melt bath immersion temperature of 465° C. into the melt bath, contained in a housing enclosure under a protective gas atmosphere containing a maximum of 10 ppm oxygen. The melt bath consisted of a Zn melt, which as well as unavoidable impurities (e.g. Fe contents, which are drawn into the melt bath by the strip), contained 0.2% by weight Al, and 0.8% by weight Mg. Immersion time was two seconds.

After being conducted out of the melt bath, the coating thickness on the steel strip applied on both sides was adjusted, while still inside the melt bath housing enclosure, by means of stripper nozzles, likewise arranged in the housing enclosure, to a coating thickness of 3 μm on each side (corresponding to a coating weight of 21 g/m² per side). The stripping was likewise carried out by means of nitrogen gas.

To conclude, the steel strip was dressed. The melt bath-coated steel strip obtained had Ra values of 1.8 μm, with Pc values of 46 cm⁻¹ determined in accordance with the StahlEisen Test Datasheet SEP 1940.

The ball impact hardness test in accordance with the StahlEisen Test Datasheet SEP 1931 was carried out on specimens taken from the finished coated steel strip in order to determine the adherence of the coating and its formability. The result could be classified as Stage 1, which corresponds to good adherence and likewise good ability to forming.

In a deep drawing test, in which a hat-shaped component was drawn in a suitable mould out of a round steel sheet blank, a very low friction value was achieved, of a maximum of 0.45 g/m².

The evaluation of weldability produced, for a laser-beam welded seam, a very good result in each case. Thus, for example, with laser beam welding with a joint gap “0” with welding speeds of up to 5 m/min, error-free results are achieved.

The salt spray test in accordance with DIN 50021 SS carried out on an unpainted bright specimen, coated in the manner explained above, did not have first red rust formation until after a spray duration of 312 hours. With a conventional Zn coating on a sheet with a coating weight of 25 g/m² per side, red rust formation already appeared after 24 hours.

The paint adherence on samples coated according to the invention, in the conical mandrel bending test based on DIN EN ISO 6860, was good both at room temperature as well as at −20° C. The stone impact test according to DIN 55996-1B likewise did not result in any flaking of the coating from the steel base layer. 

1-5. (canceled)
 6. A method for the manufacture of a steel sheet or strip coated on at least one of side with a coating of 0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, the remainder being zinc and unavoidable impurities, the coated steel strip, in a salt spray mist test carried out in accordance with DIN 50021-SS, shows the first red rust formation at the earliest after 250 hours, the method comprising: annealing an uncoated steel strip; conducting the annealed uncoated steel strip through a galvanizing bath including 0.05-0.30% by weight Al and 0.2-2.0% by weight Mg, the remainder being zinc and unavoidable impurities, to form the coated steel strip; and adjusting a coating layer thickness and a coating weight on the coated steel strip emerging from the galvanizing bath using a stripper nozzle; wherein (i) a surface of the galvanizing bath is kept under an inert gas flow in relation to the surrounding atmosphere, to inhibit formation of slag or inter-metallic phases; and (ii) the coating layer thickness is up to 3.5 μm on each side and the coating weight is up to 25 g/m² on each side.
 7. The method of claim 6 wherein the coating includes 0.4-1.0% by weight Mg.
 8. The method of claim 7 wherein the coating includes more than 0.5% by weight Mg.
 9. The method of claim 6 wherein the Al content of the galvanizing bath amounts to 0.12-0.14% by weight.
 10. The method of claim 7 wherein the Al content of the galvanizing bath amounts to 0.12-0.14% by weight.
 11. The method of claim 8 wherein the Al content of the galvanizing bath amounts to 0.12-0.14% by weight.
 12. The method of claim 6 wherein the Al content of the galvanizing bath amounts to at least 0.15% by weight.
 13. The method of claim 7 wherein the Al content of the galvanizing bath amounts to at least 0.15% by weight.
 14. The method of claim 8 wherein the Al content of the galvanizing bath amounts to at least 0.15% by weight. 